powerpc64: fix the calculation of Maxmem

The calculation of Maxmem was skipping the last phys_avail segment,
because of a wrong stop condition.

This was detected when using QEMU/PowerNV with Radix MMU and low
memory (2G). In this case opal_pci would allocate a DMA window that
was too small to cover all physical memory, resulting in reading all
zeroes from disk when using memory that was not inside the allocated
window.

Reviewed by: jhibbits
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D33449
MFC after: 2 weeks